This invention relates to optical amplifiers and more specifically to a method for automatic gain, pump power or current and output power control, and ASE compensation of Optical Amplifiers.
In recent years optical amplifier modules have undergone considerable transformation. Increased demand for more data transfer resulted in development of wavelength division multiplexing (WDM) technology, which allows more data to be transmitted over one fiber by increased channel count (i.e., a larger number of narrower wavelength ranges within the same predetermined wavelength window). This WDM technology suffers from unwanted effects, such as a variation in output power when the input signal power is constant (for example, due to aging of the amplifier or due to stresses in the amplifier), and cross talk between different channels, for example, when the input signal is modulated at a low frequency. The low frequency is a frequency of up to 10 kHz. This low frequency modulation can be present, for example, due to the addition or dropping of some to the channels, or due to sudden loss of signal at certain wavelengths. These unwanted effects have a negative influence on the power transients (i.e., fluctuations of output optical signal power) of surviving channels, which results in a poor performance of the signal transmission, expressed in an increased bit error rate (BER).
In order to minimize the unwanted output signal power fluctuation and the power transients due to the cross talk or other causes (such as fiber damage, adding or dropping of channels), it is common to introduce a mechanism for controlling either the output signal power or the gain of the optical fiber amplifier. Gain is the ratio of the optical signal output power to the optical signal input power.
There are two known approaches for controlling output signal power or the gain of the optical fiber amplifier. The first approach, known as the electronic feedback/feed-forward approach, utilizes electronic circuitry to control power transients caused by the crosstalk produced in the optical fiber amplifier. More specifically, amplifier gain or power is controlled by analog tuning of the electronic components, for example by changes resistor""s or capacitor""s values. This approach allows the user, such as a communication company, to minimize power transients in any given optical amplifier by controlling either the amplifier gain or the amplifier output power, but not both. This approach also limits accuracy of gain control when signal power is small. Finally, this approach does not compensate for amplifier noise, such as ASE (amplified spontaneous emission).
The second approach, known as the optical feedback control approach, utilizes only optical components to control power transients of the optical fiber amplifier. This approach is even less flexible than the all-electronic approach described above, because any change in power or gain control requirements requires the change in optical components.
The present invention is set forth in the appended claims. According to one embodiment of the present invention a method for compensating for ASE in gain control mode while controlling gain of an optical amplifier, the method including the following steps: (i) obtaining value for input and output power electronic circuitry DC offset; (ii) selecting the following set point values: power set point=0.0, gain set point=desired gain, (iii) obtaining digital value representing signal input value and subtracting input power electronic circuitry DC offset to produce adjusted input power value; (iv) obtaining digital value representing signal output value and subtracting output power electronic circuitry DC offset to produce adjusted output power value; (v) obtaining the ASE content of the adjusted input power value by either utilizing a lookup table or a formula and subtract ASE content from the adjusted input power value to produce ASE adjusted output power value; (vi) either multiplying the gain setpoint by the obtained ASE adjusted input power value and subtracting this from the ASE adjusted output power value, or dividing the ASE adjusted input power to obtain gain control error signal; (vii) subtracting the gain control error signal from any number to produce a controller input signal; (viii) transforming the controller input signal into digital representation of at least one pump control signal by a control algorithm; (xi) transforming said digital representation of said at least one pump control signal into at least one corresponding analog pump control signal; and (x) applying the at least one analog pump control signal to at least one pump via driver circuitry.
For a more complete understanding of the invention, its objects and advantages refer to the following specification and to the accompanying drawings. Additional features and advantages of the invention are set forth in the detailed description, which follows.